1. Field of the Invention
The present invention relates to a photothermographic material.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal developing image forming systems utilizing organic silver salts are known. In particular, photothermographic materials generally comprise an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. The Fuji Medical Dry Imager FM-DPL is an example of a practical medical image forming system using a photothermographic material that has been marketed.
Methods of manufacturing such a photothermographic material include a method of manufacture by a solvent coating, and a method of coating an aqueous coating solution using an aqueous dispersion of fine polymer particles or an aqueous solution of a water-soluble polymer as a main binder followed by drying. Since the latter method does not require a process of solvent recovery or the like, a production facility therefor is simple, environmental burden is small, and the method is advantageous for mass production.
In the case of the photothermographic material having an aqueous coated image forming layer, methods of using a hydrophobic latex as a main binder for the image forming layer in order to prevent moisture from influencing photographic performance are described in Japanese Patent Application Laid-Open (JP-A) Nos. 10-10670, 10-186568, and 2000-227643. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. However, since the obtained images are usually handled and stored under various environmental conditions, image stability and image quality must be maintained under any environmental conditions. Nevertheless, the above materials have not reached the level of conventional wet developing silver halide photosensitive materials with regard to, for example, their resistances to grazing and scratches. The conventional wet developing silver halide materials include an additive, called a hardener, which can crosslink gelatin chains of gelatin binders to harden their membranes to improve resistance to water and resistance to scratches. However, until now photothermographic materials could not realize a satisfactory level of water resistance and scratch resistance. Maintaining high image quality while attaining good image storability is a very important and difficult task, which therefore continuously requires new technical development.